The present invention relates to an electrophotographic type color image forming apparatus and, more particularly, to a color image forming apparatus using a liquid developer with a fine particle coloring material mixed into an insulating solvent.
Of the electrophotographic type image forming apparatus, a so-called wet type image forming apparatus using a liquid developer has advantages not realized by a dry type image forming apparatus and, in recent years, their merits have been reevaluated. The wet type image forming apparatus has major advantages over the dry type image forming apparatus in terms of (a) being capable of realizing a high image quality with the use of a very fine particle toner of the order of a submicron size, (b) capable of obtaining an adequate image density with less amount of toner and, in addition to being thus economical, also capable of realizing a texture just equal to that of a printing (for example, an offset printing), (c) capable of fixing the toner to a recording sheet at a relatively low temperature and thus achieving an energy saving, etc.
In the conventional wet type electrophotographic technique using a liquid toner as a developer, however, there arise essential problems and, for this reason, the dry type electrophotographic technique has been unrivaled in its market. As its one problem, it has to use a liquid developer containing a high resistant or insulating petroleum solvent as a carrier solvent so as to develop an electrostatic latent image. In an image forming process, this solvent is quickly consumed and it is, therefore, necessary to effect a frequent exchange of the solvent tanks, so that the maintenance becomes poor.
For this reason a squeegee roller is provided so as to, after a developing step, scrape any excessive solvent from a photosensitive drum surface. It is thus necessary to collect any not-requisite solvent, even if being small in its amount, from the drum surface for a recycling purpose. Since, at the same time, there arise an evolution of an odor resulting from the evaporation of the solvent as well as an allergy action of it on a human body, etc. It is, therefore, desirable to collect the solvent into the apparatus to a practically possible extent and prevent it from being evaporated to an outside.
In the image forming apparatus using a liquid developer, an indirect transfer method has usually been employed according to which, in order to retain a better image, an image on a photosensitive drum is once transferred to a transfer roller, such as a transfer drum, instead of effecting an image transfer in a way to set a recording sheet in direct contact with the photosensitive drum, and an image on the transfer roller is transferred to a recording sheet by contacting the image with the recording sheet under an application of a pressure to the recording sheet. In this case, the image on the photosensitive drum is placed under an electric field or a pressure such that the image can be readily moved onto the transfer drum.
In such a process, the solvent is evaporated on a heated transfer drum and, when the evaporated solvent (solvent vapor) is exhausted to an outside of the apparatus, the odor and allergy problems may occur as set out above. It is, therefore, necessary to prevent the diffusion of such solvent vapor produced on the transfer drum.
In the image forming apparatus using a liquid developer, it is required that, while maintaining the conditions of forming a good quality image, consideration be paid to ensuring an effective consumption of the developer and environmental health. It is, therefore, necessary to prevent any leakage of the solvent vapor from the apparatus and to collect the solvent vapor, through liquefaction, for a recycling purpose. The greatest task of the wet type image forming apparatus using the liquid developer is as to how the developer solvent should be recovered for recycling and how the leakage of the solvent to an outside be suppressed to a minimal extent.
Against these requests a method may be considered by which the solvent vapor is reduced by passing air in the apparatus, for example, through a filter. In this case, the solvent vapor collection method is by introducing a circulation air in the apparatus into a filter using an adsorbent such as an activated carbon and, after adsorbing the solvent vapor onto the adsorbent, returning it again into the apparatus or exhausting it to an outside, etc. Since, in this method, no entire consideration is paid to the recycling of the solvent vapor, much more solvent is consumed and, since the life of the filter is usually limited, it is determined depending upon an amount of adsorbing solvent. Therefore, the greater the amount of solvent vapor, the quicker the filter's performance begins to be lowered. This necessitates the frequent exchanges of filters. Further it may be considered that a scattered solvent vapor in the apparatus is deposited here and there and gives a serious damage to the apparatus.